Casting flux

ABSTRACT

The invention relates to a casting flux for steels or alloys on an iron, nickel or cobalt basis which make heavy demands on the degree of oxidic purity for continuous or ingot casting and contain as the main components calcium oxide (CaO), aluminium oxide (Al 2  O 3 ) and strontium oxide (SrO), the characterizing feature of the invention being that the chemical composition lies within the following limits (in % by weight): 
     20 to 40% CaO, 
     15 to 30% SrO, 
     0 to 6% Mgo, 
     0 to 8% MgF 2 , 
     0 to 8% CaF 2 , 
     0 to 8% NaF 
     0 to 6% LiF 
     residue Al 2  O 3 , 
     the flux having a total content not exceeding 15% of oxygen-yielding compounds, such as SiO 2 , FeO, MnO, K 2  O, Na 2  O, P 2  O 5 , Cr 2  O 3  and B 2  O 3 .

The invention relates to a casting flux for steels or alloys on an iron,nickel or cobalt basis which makes heavy demands on the degree of oxidicpurity for continuous or ingot casting. The term casting flux in thiscase also includes powders for the capping and after-treatments of metalmelts in ladles or intermediate vessels.

The casting fluxes hitherto used in practice are built up on a silicatebasis, containing as main component 20 to 40% by weight SiO₂, inaddition to CaO and Al₂ O₃. In connection with Na₂ CO₃ and CaF₂ and insome circumstances B₂ O₃, in addition to other important properties thelow melting temperature required for casting is set below 1200° C., thenecessary viscosity being in the range of approximately 1 Pa.s, with avitreous state at temperatures below 800° C. In addition these castingfluxes also contain other oxides, such as iron and manganese oxide andalso P₂ O₅, which are introduced via the raw materials. In some casesthey are also deliberately added to obtain the aforementioned propertiesto the required extent. Casting fluxes are also used in industry whichin order to maintain a vitreous solidification up to as low temperaturesas possible contain increased SiO₂ contents with a low CaO/SiO₂ ratiobelow 1.0, to prevent crystalline precipitations, for example, cuspidinor nephelin, from the vitreously solidifying casting slag in the castinggap.

Due to their relatively low thermodynamic formation energy, thesecasting fluxes on a silicate basis with additions of Na₂ CO₃ and in somecases B₂ O₃ and also iron and manganese oxides have a considerableoxidation potential in relation to steels and alloys on an iron, cobaltand metal basis with a low oxygen content. Reaction with alloyingelements, such as aluminium, titanium and others causes non-metallicinclusions in the solidified metal due to which the degree of oxidicpurity and therefore the properties of use of these metals mayconsiderably deteriorate. Hitherto there has been no technicallyfeasible way of achieving the necessary low oxidation potential of thecomponents of the casting flux without abandonment of the componentshitherto used, which more particularly effect vitreous solidificationdown to low temperatures.

U.S. Pat. No. 3,926,246 discloses the addition of controlled proportionsof alkali metal oxides and phosphorus pentoxide in addition to thecomponents normally found in casting fluxes, such as fluorides, alkalineearth oxides, aluminium oxide, silicon oxide, lithium oxide and boronoxide. The result is a substantial and in the case of certaincompositions a complete vitrification of the casting flux slag, whilemaintaining flowability, softening behaviour and aluminium oxideabsorptivity. However, although the very high additions of alkalioxides, phosphorus pentoxide, silicon oxide and boron oxide, forexample, 18-24% Na₂ O or 40% P₂ O₅ and 25% SiO₂ alongside 20% P₂ O₅ensure the required vitrification of the casting slag, while maintainingthe other aforementioned properties, they lead to a heavy yield ofoxygen from the casting slag to the liquid steel, thereby causing aconsiderable deterioration in the degree of purity of the continuouslyor ingot cast steel by the formation of non-metallic inclusions.

Similarly to the known casting fluxes, known distributor capping bodiesand ladle stopper slags consist of silica or basic oxides and, just likethe casting fluxes, have a considerable oxidation potential in relationto steels and alloys on an iron, cobalt and nickel basis with a lowoxygen content. Thus, when these ancillary materials are used, thereaction with the alloying elements, such as aluminium, titanium,non-metallic inclusions contained in the steel produces in the liquidmetal inclusions which enter the chill mould during the subsequentcasting process and lead to a contamination of the metal.

In contrast, it is an object of the invention to develop a metallurgicalancillary material in powder form which has a reduced oxidationpotential in comparison with the known ancillary materials, butnevertheless meets the demands made on the slags used in the productionof steel.

This problem is solved according to the invention by a casting fluxwhich has

20 to 40% CaO,

15 to 30% SrO,

0 to 6% MgO,

0 to 8% MgF₂,

0 to 8% CaF₂,

0 to 8% NaF

0 to 6% LiF

residue Al₂ O₃,

and has a total content not exceeding 15%, preferably not exceeding 5%,of oxygen-yielding compounds, such as SiO₂, FeO, MnO, K₂ O, Na₂ O, P₂O₅, Cr₂ O₃ and B₂ O₃. According to the invention the total content ofthe oxygen-yielding compounds must not exceed 15%, since otherwise atransfer of oxygen from the casting slag to the metal melt takes place,resulting in the formation of undesirable non-metallic inclusions in thesolidified metal alloy.

BRIEF DESCRIPTION OF THE DRAWING

The sole drawing FIG. 1 is a diagram depicting a ternary system of maincomponents CaO, Al₂ O₃ and SrO of the casting flux. The diagram includesa hatched area.

In the case of metals which are particularly sensitive to non-metallicinclusions, such as aluminium-killed deep-drawing quality steels forouter skin components or metals with alloying components having a highaffinity for oxygen, such as titanium-stabilized austenitic steels, thetotal contents-of oxygen-yielding compounds in the casting flux must belimited to a maximum of 3%.

Normally various amounts of carbon are added to the mixture according tothe invention, in dependence on the casting process.

The invention substantially dispenses with the addition ofoxygen-yielding additives, without any adverse effect on vitrificationand the other standard properties of casting flux. The limitation of thecompounds even produces a stable vitreous state during cooling. It mustbe specially pointed out that by the composition according to theinvention, vitrification is achieved without alkali oxides, B₂ O₃ andSiO₂. Alkali, iron and manganese oxides have a high oxygen potential incomparison with the other oxygen-yielding oxides, so that it isconvenient to limit each of these compounds to no more than 5%, butpreferably no more than 2%.

As already stated, more particularly when the ancillary material is usedin the form of a casting flux, it is very important to maintain thevitreous state of the casting slag in the casting gap between the chillmould and the solidified strand shell, without the possibility ofcrystalline precipitations forming which cause faults in the strandshell. This can be done particularly successfully if the chemicalcomposition of the three main components CaO', Al₂ O₃ ' and SrO' lies inthe hatched area of the ternary system shown in FIG. 1. Thisvitrification could not be readily expected, since it occurs only to avery limited extent in lime-aluminate melts. The addition of very lowSiO₂ contents can appreciably enhance vitrification withoutsubstantially raising the oxygen potential. This is more particularly ofgreat importance, since hitherto the vitreous state of the casting slagshas been possible only on a silicate basis.

The invention will now be explained by an example of comparison betweena known casting flux and a casting flux according to the invention(Table 1).

                  TABLE 1                                                         ______________________________________                                                                Example according                                                 Comparison Example                                                                        to the invention                                                  % by weight % by weight                                           ______________________________________                                        SiO.sub.2     35.5          0.3                                               CaO           23.5          26.9                                              Al.sub.2 O.sub.3                                                                            6.0           27.0                                              MgO           0.9           3.1                                               Na.sub.2 O    5.0           0.2                                               CaF.sub.2     11.1          4.0                                               Fe.sub.2 O.sub.3                                                                            1.1           0.2                                               C uncombined  4.5           5.5                                               SrO                         21.1                                              MgF.sub.2                   3.7                                               NaF                         3.5                                               LiF                         2.3                                               FeO                         0.1                                               MnO                         0.1                                               Annealing loss                                                                              12.4          2.0                                               Liquidus temperature (°C.)                                                           1187          1162                                              Viscosity (Pa.s)                                                                            0.73          0.15                                              at 1300° C.                                                            ______________________________________                                    

Using the two casting fluxes, aluminium-killed deep-drawing qualitysteel for the outer skin parts of motor cars having the followingprescribed chemical composition: max. 0.04% C, 0.15 to 0.22% Mn, 0.030to 0.050% Al_(sol). was continuously cast in the form of slabs in asequence of 300 t melts each, rolled into cold rolled coils andinvestigated during inspection for faults close to the surface due tothe casting techniques. In the case of the coils originating from themelts cast with the casting flux according to the invention, rejectionsdue to outer skin part faults were reduced to one fifth of the qualityfaults found in parts cast using the known casting flux. In addition tothe higher profit to the steel manufacturer, this means that furtherprocessers have reduced storage costs.

We claim:
 1. A casting flux contains CaO, Al₂ O₃ and SrO suitable foruse with steels or alloys based on iron, nickel, or cobalt, consistingessentially of, in % by weight,20 to 40% CaO, 15 to 30% SrO, 0 to 6%MgO, 0 to 8% MgF₂, 0 to 8% CaF₂, 0 to 8% NaF, 0 to 6% LiF, balance Al₂O₃,said flux having no more than 15% by weight of an oxygen-yieldingcompound selected from the group consisting of SiO₂, FeO, Fe₂ O₃, MnO,K₂ O, Na₂ O, P₂ O₅, Cr₂ O₃, B₂ O₃ and combinations thereof.
 2. Thecasting flux of claim 1 having no more than 5% by weight of saidoxygen-yielding compound.
 3. The casting flux of claim 1 having no morethan 3% by weight of said oxygen-yielding compound.
 4. The casting fluxof claim 1 wherein the alkali, iron, and manganese oxide contents areeach no greater than 5% by weight.
 5. The casting flux of claim 1wherein the alkali, iron, and manganese oxide contents are each nogreater than 2% by weight.
 6. The casting flux of claim 1 having an SrOcontent of 15 to 20% by weight.